Anaerobic digestion is one system currently used as a sustainable way to deal with waste such as manure, crop residues, protein rich substrates, etc. Livestock manure has significant resource potential. It is a valuable source of crop nutrients and also represents a substantial bioenergy resource if processed by anaerobic digestion.
Using oxygen-free conditions, an anaerobic digester typically uses microorganisms to transform organic waste into biogas and soluble nutrients among others. Raw biogas typically consists of methane, carbon dioxide, water vapor, trace amounts of hydrogen sulfide and nitrogen. The biogas produced may be converted to electricity or collected and used as a direct energy source. Biogas may also be upgraded and fed into natural gas pipelines. Unlike fossil fuels, use of renewable resources represents a closed carbon cycle and therefore does not contribute to increases in atmospheric concentrations of carbon dioxide. In effect, anaerobic digestion is a carbon dioxide neutral solution.
The most common type of digester found on a farm is a complete mix digester. This digester is typically composed of a large round insulated concrete tank that sits above ground. A plastic membrane covers the tank in order to collect the produced biogas. The digester necessitates the use of liquid or slurry like substrates that contain 3% to 15% solids.
The complete mix digester functions with a continuous addition of substrate to the reactor. Once the substrate is in the digester, it is periodically agitated by a motorized mixer or a liquid recirculation pump. This helps to keep the solids in suspension, thus increasing the digester's efficiency. The complete mix digester supplies an eternal amount of biogas as long as new substrate is always added to the digester. This technology is suitable for slurries with a moisture content between 85% and 97%. However, substrates with large amounts of fiber or higher solid content are unsuitable because agitation becomes impossible or very energy intense.
Another type of digester is referred to as a batch digester and functions with batches of biomass. Typically, the batch digester has the same structural composition as the complete mix digester, however, there may or may not be agitation motion inside the digester. A large amount of waste such as manure is added to the digester at one time and it remains sealed until the end of the digestion process. After the digestion process it is removed and treated for other applications. The average retention time for a batch digester is typically 10 to 20 days. This enables the digestion to take place and sufficient biogas to be produced. However, the microbiology of the digester needs to constantly adapt to new environmental conditions, which makes the system less efficient.
Another type of digester is referred to as a plug flow digester and is typically composed mainly of a large rectangular tank that is dug into the ground. The tank is much longer than it is wide, typically approximately a 5 to 1 ratio, and is covered with an expandable plastic cover to retain the produced biogas.
Typically, the best substrate for the plug flow digester is manure with a sufficiently high solid content, usually from 11% to 14%. Most farmers therefore use scraped manure devoid of much of the bedding and containing substantially no sand. The plug flow digester typically does not have any internal agitation and instead, the manure flows through the digester. The constant addition and withdrawal of manure allows this displacement. However, there is always the possibility for some manure that floats on top or travels through the digester faster and some manure that settles to the bottom and remains in the digester. Usually the manure remains in the digester for 15 to 20 days before it is removed. Like other digesters, the plug flow digester collects the biogas produced by the manure. The biogas can then be extracted from the digester and used to generate electricity and heat. Again with a plug flow digester, the microbiology needs to constantly adapt to new environmental conditions, which makes the system less efficient.
One process that treats solid substrates is referred to as the garage type digester. These digesters consist of a large air-tight compartment that typically resembles a garage. The substrate contained in the digester is composed of 30% or more solid content. The addition of small amounts of fresh crops and wood to the substrate is also possible.
The solid substrates are added to the compartment in batches. Once filled, the digester will not be opened until the digestion process is complete. The substrate is lightly and equally sprayed with water from above. The water percolates through the biomass, digesting it and producing biogas. The water is then collected at the bottom of the compartment and pumped into a temporary storage tank. This water is used to continuously spray the biomass.
The retention time for this digester is longer than others, averaging at about 28 to 30 days. The temperature inside the digester is usually maintained at around 40° C. The biogas is collected at the top of the compartment. One downside of this process is that moving the substrates into the garage and out of it again is very labour intense. Further, storage is required for the digested material.
Another type of digester is referred to as a dry digester. The dry digester takes many forms. However, the predominant design remains a type of plug flow digester oriented in a vertical or horizontal position.
The substrates used for dry digesters can contain up to 15% to 45% solids. Farmers therefore often use a mix of silaged crops and solid manure to fill the digester. In the case of a vertical dry digester, such as the one built by DRANCO-FARM technology in Nünstedt, Germany, the biomass is added at the top and travels towards the bottom due to gravity only. The produced biogas is collected at the top of the digester. In a horizontal dry digester design, the biomass moves from one end of the tank to the other propelled by motor driven agitators. There is also a conveyor belt at the bottom of the tank that picks up any sediment. Usually the retention time for a dry digester is only 2 to 3 days due to the faster rate of displacement. These systems tend to be expensive and require mechanically moving parts exposed to the biogas.
A need exists to provide an anaerobic substrate treatment device, system and/or method that overcomes or mitigates at least one of the problems outlined above and/or is observed in the field.